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Mineral Processing

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Mineral Processing …

Mineral Processing
Jaw crusher
gyratory Crusher
beneficiaton
roll crusher
screening separation classifier
grinding crushing law dry grinding wet grinding Ned university My-203

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  • 1. 2
  • 2. 3 (Ore)
  • 3. 4 WHAT CONSTITUTES MINERAL BENEFICIATION? Ore is an aggregate of minerals and contains valuable and gangue minerals . The mineral beneficiation involves separation of gangue minerals from ore. It has three steps Liberation , Separation, Concentration. 1) Liberation of valuable mineral by size reduction. 2) Separation of coarse and fine particles. 3) Concentration to separate the gangue minerals to increase the metal grade. If the first step is not done correctly, the second step will be incomplete.
  • 4. LIBERATION BY SIZE REDUCTION. Consider a cube ( 10 cm dia )of the ore having MINERAL and GANGUE. Suppose it has cubic grains of 10 mm. Assumptions 1. crushing is conducted to yield grains of same size. 2. crystals in the ore are intimately joined with each other. Till the ore lump is crushed to 10 mm dia, all grains are locked. If the ore lump is crushed to particles of 5 mm dia, some grains are FREE and some are LOCKED. It cannot be assumed that all particles are free since they are of 5 mm dia ( smaller than 10 mm dia). Locked grains 5 =10 x10 mm
  • 5. Even with heavy overgrinding, particles cannot be liberated completely. The crushed particles are obtained in the form of LOCKED MIDDLINGS. MINERALS OF EQUAL ABUNDANCE. The cube is sectioned along a vertical plane. Visualise that lattices with parameters 10mm and 5 mm are superimposed with their axes parallel. It appears that , in the plane of section, alternate rows ‘A’ and ‘C’ only have “free looking” particles. Other alternate rows ‘B’ and ‘D’ do not have them. The free particles are 1,3,5,13,15,17. ( They may be locked in other planes ). Free particles of mineral and gangue – 6 out of 48 (1/8) ( and not 6 out of 24). Degree of liberation of each constituent- 3 out of 24 (1/8). 6
  • 6. MINERALS OF UNEQUAL ABUNDANCE. The two constituents are not equally abundant. 1. The less abundant mineral is not free at all unless the particles are finer than the grain size. 2. To free the less abundant mineral, the particles must be made much finer than the grain size. 3. The more abundant mineral is always freer than the less abundant mineral. 7 bauxite LIBERATION BY DETACHMENT. If the ore lump is made of mineral grains bonded loosely, fracturing to the grain size results in complete liberation. Eg: Pebble phosphate rock MOSTLY LIBERATION NEEDS SIZE REDUCTION.
  • 7. 8 A SIMPLIFIED FLOW CHART FOR ORE DRESSING
  • 8. Comminution Communiation is one of the primary operations in mineral dressing ;takes place in a sequence of crushing and grinding processes. COMMINUTION - The whole operation of reducing the raw ore to the size required for mechanical separation or metallurgical processing. It is used to - create particles in a certain size and shape, - increase the surface area available for next process - liberate valuable minerals held within particles. Comminution Crushing Grinding • When raw materials are processed in solid state, particle size reduction is essential. The ROM ore may be as big as 3 m in size. • Minerals should be liberated before concentration. This liberation accomplished by comminution.
  • 9. 11 OBJECTIVES OF COMMINUTION. • Reduction of large lumps into smaller sizes. • Production of solids of desired size ranges. • Breaking apart valuable minerals from gangue ( liberation of valuables ). • Convenience of handling the ore and its transportation. • Preparation of feed material for different ore processing techniques ( Eg:- granular material for gravity separation, fine particles for froth flotation etc.) . Difference between crushing and grinding- Crushing ( dry) - Size reduction occurs preferentially on large fragments. Grinding (wet) - Size reduction is less selective- all pieces get ground to fine particles.
  • 10. 12 S. No Process Size reduction 1. Explosive shattering Infinite size to 1 m 2. Primary crushing 1m – 100 mm 3. Secondary crushing 100 mm- 10 mm 4. Coarse grinding 10 mm- 1 mm 5. Fine grinding 1 mm- 100 microns 5. Very fine grinding 100 microns – 10 microns 6. Superfine grinding 10 microns- 1 micron STEPS IN COMMINUTION Size reduction/Comminution Process : extremely energy-intensive . 5 % of all electricity generated is used in size reduction . Efficiency of size reduction : 1 %. Blasting can be described as the first stage of comminution carried out in the mine site in order to remove ores from their natural beds.
  • 11. 13 Energy utilisation in comminution = (Energy theoretically needed for a particular degree of size reduction ) ------------------------------------------------------------------------------------------------ ( Energy actually consumed ) Actual energy consumed = power input to the mill = energy needed to move the working parts with suitable load+ energy needed to overcome friction+ energy needed to grind away the metal from the working face+ energy needed for size reduction. A major part of the energy is lost as HEAT. ENERGY IN COMMINUTION If the particles of the ore has SINGLE MINERAL only – FREE PARTICLES. If they have two or more minerals- LOCKED PARTICLES. Normally the ore consists of at least two minerals which are intimately interlocked. DEGREE OF LIBERATION – of a certain mineral is the percentage of that mineral occurring as FREE PARTICLES w.r.t the total of the mineral occurring in the FREE and LOCKED forms. DEGREE OF LOCKING - of a mineral is the percentage occurring in LOCKED PARTICLES w.r.t the total occurring in the FREE and LOCKED forms.
  • 12. 14 • Very inefficient at creating new surface area (~1-2%) • Surface area is equivalent to surface energy • Comminution energy is 60-85 % of all energy used • A number of energy "laws" have been developed • Assumption - energy is a power function of D dE= differential energy required, dD= change in a particle dimension, D = magnitude of a length dimension, K = energy use/weight of material, and n = exponent ENERGY IN COMMINUTION n DK dD dE  Rittinger’s, Kick’s and Bond’s theories are used to evaluate any crushing process. Rittinger’s law deals with measurements of surface areas Kick’s law deals with volumes of product’s particles Bond’s theory deals with lengths of cracks formed. Kick applies to coarse sizes (> 10 mm), Bond applies down to 100 µm, Rittinger applies to sizes < 100 µm .
  • 13. 15 It is impossible to estimate accurately the amount of energy required to effect a size reduction of a given material. A number of empirical laws have been proposed. The two earliest laws are due to KICK and VON RITTINGER, and a third law due to BOND has also been proposed. Von Rittinger’s Law (1867) :- It states that the energy consumed in the size reduction is proportional to the area of new surface produced. E - the energy input, D1- the initial particle size, D2- the final particle size, K - a constant. Kr = Rittinger's Constant and fc = crushing strength of the material, f- feed, p-product. ) D 1 D 1 (fKE fp cr 
  • 14. 16 Kick’s law (1885) States that the work required is proportional to the reduction in volume of the particles concerned. Energy required for producing a specified reduction is proportional to the log of the reduction ratio. It is based on the assumption that geometrically similar particles would always break in geometrically similar manner, irrespective of the size.          p f eck D D logfKE Kk = Kick's Constant and fc = crushing strength of the material
  • 15. 17 Bond’s law (1952) Energy required is based on geometry of a crack expansion as it opens up. Bond has developed an equation which is based on the theory that the work input is proportional to the new crack tip length produced in particle breakage, and equals the work represented by the product minus that represented by the feed. The total work input (represented by a given weight of crushed or ground product ) is inversely proportional to the square root of the diameter of the product particles. Kb = Bond's Constant and fc = crushing strength of the material Fracture of the ore lump occurs by the extension of existing cracks. Such cracks lower the activation energy of fracture. ) D 1 D 1 (fKE fp cb 
  • 16. 18 LIMITATIONS OF COMMINUTION LAWS. The size and shape which can be determined by these methods is not very perfect. It is determined on the average method . The methods discussed above can’t measure the accurate area of the particles. So results may not be accurate. It is very difficult to determine accurate area of the finer particles by these methods. There is no suitable method available for the determining the area of the fine particles. Even if the above conditions are determined, the surface areas of cracks present in big particles would remain unaccounted. If all the above conditions are considered then the area of the cracks which are present in the particles can’t be calculated. It is the major drawbacks of the calculating the area of the particle surface.
  • 17. 19 OPERATING VARIABLES Comminution of the ROM ore is affected by 5 variables. 1. Moisture content of solids. Moisture < 3-4 % by wt. no difficulties. Moisture content > 4wt%- Ores become sticky, clog the machines. Excess moisture ( > 50 wt%) –Easy to feed and remove the product from the size reduction area. Size reduced minerals are easily transported as slurry. Grinding is normally done in wet condition ( Crushing is a dry process). 2. Reduction ratio ( RR ). Average diameter of the feed / average diameter of the product. In primary crushers , RR = Gape / Set. Gape- Size of the receiving opening ( max size acceptable in the machine). Set - Size of discharging opening ( max. size passing through the discharge end). Normal RR - 3-7 ( for coarse crushing ), - as high as 100 ( for fine grinding). Energy requirement increases with RR.
  • 18. 20 3. Free crushing. Individual particles are crushed freely. The crushed product is removed from the crushing zone quickly. It avoids formation of excessive amounts of ‘fines’ ( the number of contacts are less, hence less crushing). 4. Choke feeding. It is the reverse of Free crushing. The feed hopper of the crusher is always full ( choked) . It prevents complete discharge of the crushed products. It increases the amount of ‘fines’. Preferred when process becomes more economical ( one or more intermediate crushing stages can be dispensed with).
  • 19. 21 5.Closed / open circuit operation. Oversized material is returned to the size reduction unit for further reduction after sizing- closed circuit- more economical, more uniform product. If no material is returned- open circuit. Closed circuit Open circuit Crusher
  • 20. 22
  • 21. Crushing The feed size varies from 1.5” to 6” Primary Secondary CrushingCrushing Jaw crushers & Gyratory crushers Cone crusher & Roll crusher The feed size varies from 3” to 30”
  • 22. 24 CRUSHING THEORIES Crystal structure is bound together by its inter-atomic forces of attraction . Disruption of these bonds needs stress = theoretical strength of the crystal. Theoretical strength of a crystal = 7 X 104 Kg/cm2. When the applied tensile load exceeds the elastic limit, a flaw ( defect ) is produced in the crystal. The stress which the crystal can withstand depends on the length of the crack ( σ ( √ length of crack ) – GRIFFITH’S LAW ) i.e., the product of the square root of the flaw length (a) and the stress at fracture (σf) was nearly constant, which is expressed by the equation: i.e, fracture occurs at a much lower stress loading in the presence of a crack. Once sufficient energy is given, the crack propagates very fast ( 5000 m/sec). α
  • 23. 25 Most ROM (run of mines) ores have such fine cracks. So crack propagation becomes very easy at lower crushing loads. There is a critical value for the crack length at any particular level of stress . Now, the increased stress level at the crack tip is sufficient to break the atomic bond at that point. Such rupture of the bond will increase the crack length. It increases the stress concentration and causes a rapid propagation of the crack through the matrix. This leads to final fracture. When fracture occurs, some of the stored energy is transformed into free surface energy ( the potential energy of atoms at the newly produced surfaces ). REDUCTION RATIO IN CRUSHING - for comparing the performance of various crushing machine- Max. size of the particles in the Feed / max. size of particles in the product.
  • 24. 26 Crushing occurs due to one or a mixture of the disruptive forces Compression Tension Shear Torsion Impact Shatter
  • 25. 27 MECHANISM OF CRUSHING. COMPRESSION /SHEAR. Crushing results from stresses that are applied to the particles to be crushed. Crushing is done by a moving part, working against a stationary / another moving part. The stresses setup strains within the particle. When the strain exceeds elastic limit, the lump fractures along principle shear stresses into large and fine particles. If the crushing force is not sufficient, no crushing occurs.The material may deform. THAT IS THE REASON TO USE RUGGED, MASSIVE CRUSHERS IN MINERAL DRESSING.
  • 26. 28 The best method to cause rapture in solid materials is the application of shearing loads. The orientation of crystals in ores is generally irregular. So, compressive force is sufficient. The SIZE REDUCERS AS CRUSHERS USE COMPRESSION, SHEAR OR BOTH AS FRACTURING FORCES.
  • 27. 29
  • 28. Primary crushers Jaw crushers Gyratory crusher PRIMARY CRUSHING - Reduction of ore = as mined size ~1m to ~ 100 mm size. The objects of crushing are 1) size reduction 2) production of a minimum amount of fines. Primary crushing is done on dry material. The basic equipments for primary crushing- 1) Jaw crushers. 2) Gyratory crushers. 3) Roll crushers.
  • 29. 31 JAW CRUSHER- It has two crushing faces - JAWS. The jaws are made of cast steel. They are fitted with replaceable liners made of manganese steel or Ni-hard (a Ni-Cr alloyed cast iron). They are mounted on a rigid frame. One jaw is stationary , the other is movable. It moves towards and away from the stationary jaw by a small throw. The feed ore is squeezed till it breaks. The fragments move down to a narrow part of the wedge. They get squeezed repeatedly. The final products escape through the narrow gap at the bottom. Jaw crushers are classified based on 1) the point of minimum amplitude of motion on the moving face, 2) the way of transmitting the movement to the immovable face.
  • 30. Jaw crushers:
  • 31. 34 1. BLAKE JAW CRUSHER ( 1858 )- Very large in size. The movable jaw is hinged at the top- the greatest amplitude of motion is at the bottom of the crushing face. The parts are 1. Cast steel frame, 2. Cast steel jaws with replaceable Hadfield steel ( 13wt% Mn) linings, 3. The movable jaw is pivoted at the top and operated by the eccentric, Pitman and toggles. 4. Angle between jaws- 20o.
  • 32. 35 Pit man The pit man is nearly vertical and the toggles nearly horizontal. One of the toggles is mounted in steel bearings at one end of the frame, the other end on the pitman. One end of the other toggle is set in steel bearings on the back of the movable jaw and the other end on the pitman. The rotation of the drive shaft causes an up-and – down translation of the pitman . This results in an increase and decrease of the distance between the back of the movable jaw and the frame. The movable jaw is pressed against the toggles by tension rods and Springs. CRUSHING IS EFFECTED ONLY WHEN THE MOVABLE JAW MOVES TOWARD THE FIXED JAW . ON ITS BACKWARD MOVEMENT, THE CRUSHED PRODUCT GETS DISCHARGED.
  • 33. 36 FACTORS INFLUENCING THE OPERATION OF A BLAKE JAW CRUSHER. 1. The reduction ratio ( Gape / Set) 2. Speed and amplitude of strokes and movable jaw. 3. Size distribution of feed. 4. Crushing properties of ore. CAPACITY OF A JAW CRUSHER T ( Tones/ hr) = 3.75 LS. L- Length of feed opening ( cm). S- Length of set opening ( cm) . ENERGY REQUIREMENTS OF A JAW CRUSHER. Depend on Size of the feed, Size of the product, Capacity of the crusher, Characteristics of the ore, Percentage idling time.
  • 34. 37 A DOUBLE TOGGLE JAW CRUSHER
  • 35. 38 2. TELSMITH JAW CRUSHER . It is similar to the Blake Jaw Crusher w.r.t the motion of the movable jaw. It differs from the Blake Jaw crusher in the way the motion is transmitted. The motion is transmitted to the movable jaw directly from an eccentric on the main shaft. No toggles / pitman are used. The machine is very large in size.
  • 36. 39
  • 37. 40 3. DODGE JAW CRUSHER . Here the Jaw action is reverse to that of the Blake Jaw crusher. Maximum crushing is given to the largest piece and minimum to the smallest. The fulcrum is at the lower end of the movable jaw. Only a small variation of SET occurs on the advancing and receding actions of the movable jaw. The jaw is operated through an eccentric ( Crusher has fewer parts) Advantage- constant discharge opening. More uniform product size. Disadvantage- stresses are uneven. Used mainly for laboratory applications.
  • 38. 41 4. GYRATORY CRUSHER . The crusher has two vertical, truncated conical shells. The outer shell has its apex pointing down. It is stationary. The inner shell has its apex pointing up. The inner shell ( the crushing element ) is made to gyrate - It alternately recedes from and approach all the points on the periphery of the outer shell. Tangential forces and squeezing forces crush the material. The particles rub against each other- fines too are produced. There are three types of Gyratory crushers- a. Suspended spindle type b. Supported ( fixed ) spindle type ( It is obsolete ) and c. Parallel pinch crushers. They are the HIGHEST CAPACITY MACHINES.
  • 39. Gyratory crushers
  • 40. 45 SUSPENDED SPINDLE GYRATORY CRUSHER . It has 1) An outer frame. The crusher has two vertical, truncated conical shells. 2) The frame has a wearing surface ( concave ). 3) The inner crushing head is 4) mounted on a spindle. 5) The spindle gyrates from a fixed fulcrum at the point of suspension. 6) This movement is effected by an eccentric sleeve. 7) The sleeve is attached to a gear and is rotated by a horizontal shaft .
  • 41. 46 When the crusher is unloaded, the spindle is free to rotate. When the feed is given, it is seized between the head and the concave. Now rotation stops. Gyration is the ONLY motion that causes the head to approach to and recede from the concave surface. The pressure exerted on the material in the gap. The feed is crushed and it passes through the crusher. The differential dilation in the cross section of the machine Does not allow CHOKING . The crusher gives good FREE CRUSHING. SUSPENDED SPINDLE GYRATORY CRUSHER
  • 42. 47 SUSPENDED SPINDLE GYRATORY CRUSHER. They work smoothly and continuously. Larger pieces ( ~ 2 m) can be crushed to finer sizes ( ~ 30 cm ) . Capacity ~ 5000 T / hr.
  • 43. 48 4b.PARALLEL PINCH CRUSHER. It is different from the suspended spindle gyratory Crusher. It does not make an acute cone with the apex during the gyratory motion of the crushing head. It makes a cylindrical motion. The extent of the force exerted on the feed material is same at all points along the faces of the crushing head. CHARACTERISTICS OF A GYRATORY CRUSHER. • Gyratory crushers are available for the coarsest crushing. • Capacity is much higher • It crushes continuously.
  • 44. 49 S.No CHARACTERISTIC JAW CRUSHER. GYRATORY CRUSHER. 1. 2. 3. 4. 5. 6. 7. 8. Working principle. Taper ( decrease of width between the movable and fixed crushing surfaces /meter ( or cm ) of depth- should be enough to hold the ROM ore properly . Capacity. Reduction ratio ( 4:1- 7:1). Type of material crushed. Type of feed. Gape length. Product size. Intermittent. Breaks material with its whole surface only half the time. Half of the revolution is heavily stressed- the other half is relaxed ( needs very sturdy construction) . Heavy fly wheels and high speeds of rotation are favourable for efficient operation. Less taper. Lesser. Same. Soft, clayey. Free feed. 84-120 in. Normally coarse. Continuous. Breaks the material all the time. Uniform transmission of energy. More taper. ( 35-40 cm/m). Greater. Same. Hard and brittle. Choke feed. 72 in. A range . CHARACTERISTICS OF JAW AND GYRATORY CRUSHERS.
  • 45. Which of the two following crushers we should choose for small capacity? & why? Gyratory crusher Jaw crusher or
  • 46. Jaw crusher is the better choice because of: 1-Jaw crusher can take larger size than gyratory & is better adopted to handle clayey & spongy rocks. 2-Gyartory crusher consume more power for the same feed as produces more new surfaces than jaw crusher. 3-Installation and housing costs will be more compared to jaw crushers.
  • 47. 52
  • 48. Secondary crushers Cone crusher Roll crusher is done to reduce the ore size suitable for ( wet ) grinding. The feed size will be less than 150 mm avg. dia & product size – 12 mm av.dia. The crushing machine does not have wider GAPE and very sturdy construction. Secondary crushers are arranged in series with the primary crushers. Equipments- CONE CRUSHER, ROLL CRUSHER, GRAVITY STAMPS etc. SECONDARY CRUSHING
  • 49. 54 Secondary crushers are arranged in series with the primary crushers.
  • 50. Cone crushers 1-Cone crusher is secondary crusher. 2-Product of cone crusher is the feed of grinding mill. 3-It work in closed circuit with double deck screen. 4-Product is more uniform in size .
  • 51. Cone crushers
  • 52. Merits of cone crusher over gyratory 1-Cone crusher moves faster than gyratory. 2-Rapid discharge of crushed material due to increase space from top to bottom. 3-Higher capacity and reduction ratio than gyratory crushers. 4-Work in closed circuit.
  • 53. 59 SYMON CONE CRUSHER ( 1920 ). Similar in operation to a gyratory crusher- Parts – SPINDLE, INNER CONE ( CRUSHING HEAD), INVERTED, TRUNCATED CONE (HOPPER- BOWL). Breaks rock by squeezing the rock between an eccentrically gyrating spindle and the hopper. The spindle is covered by a wear resistant mantle. It is supported in a universal bearing below the crushing head ( and not suspended). The enclosing concave hopper is covered by a manganese bowl liner. Rock enters the top of the cone crusher. It becomes wedged and squeezed between the crushing head and the hopper. Large pieces of ore are broken once. They fall to a lower position (because they are now smaller) where they are broken again. This process continues until the pieces are small enough to fall through the narrow opening at the bottom of the crusher.
  • 54. 60 Sl. No. Gyratory crusher Cone crusher 1. 2. 3. Crushing results from interaction between spindle, crushing head and bowl. The outer stationary crushing surface flares outward- increased area of discharge. The outer stationary crushing surface is held in position with a nest of heavy springs. When an uncrushable material enters , the outer crushing plate gets lifted from the lower surface and prevents fracture of the plate / damage to the machine. ( Overload protection) Same. Flares inward. Not available.
  • 55. 61 Advantages- •A cone crusher is suitable for crushing a variety of mid-hard and above mid-hard ores and rocks. •It has the advantage of reliable construction, high productivity, easy adjustment and lower operational costs. •Operated in OPEN CIRCUIT- Can be fitted with fine, medium, coarse or extra coarse crushing cavities. Speedier crushing ( 450-700 rpm ) . Reduction ratio – 3:1- 7:1.
  • 56. Roll crusher
  • 57. Roll crusher 1-Used for crushing relatively soft materials like coal & salt. 2-Capacity is high and reduction ratio is large. 3-Can not produce fine product owing to the spacing between adjacent teeth. Angle of nip . C,D Rolls E sphere of rocks N angle of nip . N E aa C D
  • 58. 64 ROLL CRUSHERS ( 1806 ) . It has two heavy cylinders. They are mounted horizontally. They revolve in opposite directions. The rolls are driven independently. The feed is NIPPED and PULLED through the rolls by FRICTION. Steel liners are provided on the rolls to avoid their damage wear. Reduction ratio is small – 3:1 – 4:1 ( since very large dia rolls cannot be made). Only small proportions of fines are produced. Better size uniformity is achieved.
  • 59. 65 ROLL CRUSHER. SET-distance between rolls
  • 60. 66 The rolls draw the ore lumps into the gap and crush between the roller faces by NIPPING ( snatching up hastily ). For a given dia. of the rolls and SET ( distance between rolls), there is a maximum size of ore lump that can be drawn in. In selecting the rolls, the sizes of the feed and product should be known. This can be computed based on the friction between the material crushed and the rolls.
  • 61. 67 Ft Fr d D Fn a/2 angle ‘a’ S A spherical particle of dia ‘d’ is positioned for crushing between two rolls of dia ‘D’. Gravity is not considered. Forces acting on the particle – at the point of contact with rolls 1) normal force Fn, 2) tangential force Ft - Resultant of the two - Fr If Fr is negative w.r.t the horizontal ( directed downward)- particle is nipped & crushed. If Fr is positive, particle will ride in the trough formed by the rolls & will not be crushed. ‘a’ Angle of Nip. – Above this the particle is seized. Above this the particle skids. S- ‘set’ – distance apart of the roll faces at the point of their nearest approach.. Coefficient of friction Ft/Fn. Vertical component of Ft – Ft cos (a/2). Vertical component of Fn- Fn sin ( a/2). ROLL CRUSHERS- ANGLE OF NIP
  • 62. 68 Ft Fr d D Fn a/2 angle ‘a’ S Under limiting conditions, Fn sin ( a/2) = Ft cos (a/2). i.e., tan a /2 = Ft/Fn. a= angle of nip. Ft/Fn- Coe. of friction φ . CONDITION TO ENSURE NIP – tan (a/2 )</= φ. Cos a/2 = [( D/2) + ( S/2)] / [ ( D/2) + ( d/2)] = ( D+S ) / ( D+d ). Max size of ore lump in relation to roll dia.( Coe of friction assumed- 0.3) Roll dia 22.5 30 60 90 120 180 cm Max size of ore gripped 0.9 1.2 2.4 3.0 4.8 7.2cm
  • 63. 69 ROLL CRUSHERS- METHOD OF FEEDING. The feed should be spread evenly. Rolls can be fed “Free” or “Choked”. Choke feeding is more efficient – higher crushing capacity. reduced wear of rolls ( ore -on-ore grinding occurs). CAPACITY OF ROLL CRUSHER. C ( tonnes / hr ) = 60 V L S ρ V = Peripheral speed ( m/mt). L = Width of the rolls ( m). S = Distance between rolls ( m ). ρ = Sp.Gravity of ore. C ~ 10-35%. Theoretically when S=0, C=0. Actually, C will be larger as choked feeding occurs and roll faces are held apart.
  • 64. 70 Another secondary crusher is toothed roll crusher. Other crushing machines are • Impact crusher. • Gyra-disc crusher. • Hammer mill. • Gravity stamps. • Squirrel cage disintegrator • Disk crusher.
  • 65. Comparison between roll & cone crushers: 1-Roll crusher production smaller of fines than cone crushers and smaller reduction ratio. 2-Rolls,because of their inability to yield high reduction ratio, have not been installed in recent years. 3-Reduction ratio for rolls should not exceed similar to cone crushers. 4-Rolls remain to be the best device for intermediate crushing.
  • 66. 72
  • 67. 73 GRINDING.
  • 68. 74
  • 69. 75 FACTORS GOVERNING THE REQUIRED ENERGY. Most of the input energy is lost as noise, heat and deformation of the ore lumps. Only a small quantity is useful in creating new surfaces i.e, grinding occurs.
  • 70. GRINDING:- Breaking down of relatively COARSE CRUSHED ore to ULTIMATE FINENESS. The fineness of the ground product depends on the subsequent mineral beneficiation operations . Theoretically, PARTICLES SHOULD BE BROKEN DOWN TILL EVERY PARTICLE IS FULLY MINERAL ( VALUABLE ) OR GANGUE. For this, fracture should be along boundaries ( detachment ) between valuables and gangue. It is not possible in practice. Hence, grinding should produce finer particles for satisfactory CONCENTRATION. LIBERATION of the mineral is to reduce the size of the particle till – individual particles are rich , the size obtained responds to subsequent concentration process. 76
  • 71. OBJECTIVES. Grinding 1) produces new surfaces 2) provides specified sizes. Different concentration methods need different sizes of feed. Concentration methods need any of the following grinding preparations. 1. For gravity/magnetic/electrostatic concentration – the feed should be coarse, overgrinding and slime should be avoided. 1. For froth flotation- upper limit of particles 200-300 μs, lower limit – 5-10 μs . 1. For chemical treatment- ( leaching ) – over grinding improves the recovery ( constituent to be dissolved should be exposed to the surface of the particles) , cost will be high. 77
  • 72. 78 OLDER TYPE GRINDING MILLS
  • 73. TYPES OF GRINDING. Depends upon the • ore. • quantity of ore to be ground. • use of the ground product. Methods • Batch or continuous grinding. • Differential grinding. • Dry and wet grinding. • Open circuit or closed circuit grinding. • Primary and secondary grinding ( stage grinding ). 79
  • 74. BATCH GRINDING. A definite quantity of the feed is ground for a pre-determined time. The ground material is removed from the mill. The grinding media will remain in the mill. Second batch is loaded for grinding and the operation continues. Tumbling mills are used. Any shape of particle ( cylindrical, cylindroconical, oval, polygonal, cubic etc ) can be used in the feed. Inefficient, more product is over ground. Used mainly for laboratory testing of ores, making paint mixtures, medicines etc. CONTINUOUS GRINDING. Ore is fed into the mill continuously at a fixed rate. Product is discharged after a suitable dwelling time. Meanwhile new feed is charged. Continuous grinding is preferred in Mineral Dressing operations. 80
  • 75. DIFFERENTIAL GRINDING. Ores and minerals vary in their relative grindabilities. Softer material may be ground finer and harder material coarser, if they co – exist. This differential action is increased in closed circuit grinding. DRY AND WET GRINDING. Material is ground either in totally dry or wet ( slurry ) condition. Grinding in moist / sticky state consumes lot of energy – grinding should not be done in this state. Dry grinding – when subsequent concentration process is dry ( concentrating ores of Cr, Au, Pb, Mn, Mo, coal..). Wet Grinding – Normally used in mineral dressing ( subsequent concentration processes - flotation, leaching etc are wet processes ). 81
  • 76. DRY GRINDING. 1. Feed material should have low ( < 1 % ) moisture content. 2. The feed should be in less contact with air ( it may absorb moisture during grinding ). 3. The grinding media and the liner inside the mill should not wear off. 4. Costly filtering equipments are not needed. 5. Dust control during grinding is needed. WET GRINDING. 1. Needs less power. 2. Needs less space. 3. Minimum dust control only needed. 4. Needs large quantities of water and good , continuous pumping system. 5. Generally cheaper than a dry grinding installation. 82 PRIMARY AND SECONDARY GRINDING. Grinding can be done in stages – primary and secondary. Primary mills work faster and shatter the feed. Secondary mills work gently and uniformly finer products are formed.
  • 77. OPEN CIRCUIT GRINDING. The mill grinds the feed to desired size in one pass and removes the product to receive the next feed. It needs more power. Needs skilled operator. Used normally - for coarse grinding, when closed circuit grinding becomes very costly. CLOSED CIRCUIT GRINDING – Mostly used in Mineral Dressing. The mill discharge is sized. Oversize particles are reground. This cycle repeats till all particles are of same size. During regrinding of oversize, new material is also fed to the grinder. Need of less skilled operators - reduced cost. FEED BALL / TUBE MILL PRODUCT FOR CONCENTRATION FEED BALL / TUBE MILL OVERSIZE CLASSIFIER PRODUCT FOR CONCENTRATION83
  • 78. 84
  • 79. 85 TUMBLING MILLS. The mill with a liner is half filled with the crushing bodies. Ore is fed at one end of the mill along with required quantity of water. Now the mill contains the feed +water + grinding media ( Crop feed / mill charge ). The ground product is discharged at the other end. The water flushes the feed through the mill. When the mill is rotated, the feed, water and the grinding media is CHURNED with FLYING ( TUMBLE ). Grinding of the feed occurs The Kinetic Energy of the tumbling load is dissipated as grinding of feed, wearing the mill lining and the media, heat and noise. The KE is usefully used for 1. Collision between particles, 2. Impact of falling grinding media, 3. Pressure loading ore particles that come under the grinding media or between the grinding media and the mill liner , 4. Shock waves transmitted when the media tumble.
  • 80. 86
  • 81. 87 TUMBLING MILL LINERS. Lined with replaceable liners made of alloy steel, rubber or ceramic materials. They may be smooth, ship-slap , wave or wedge bar type. Liners help in lifting the load as the mill revolves , to minimise the slip between layers of the grinding media and to minimise wear of the mill shell. steel liner Ceramic liner Shiplap Wave Wedge bar
  • 82. TUMBLING MILL CAPACITY. Factors influencing – specifications of the pulp ( slurry – size of solid particles ), size of the ground particles. To obtain maximum mill capacity several variables in groups are considered. 1. Group I - Mill length / mill diameter, % mill volume having the CROP LOAD ( CHARGE-ore + water + grinding media ), Number of stages – These variables will change wear of liners and grinding media. 2. Group II - Grinding media size ratio, size of new feed, solid to liquid ratio in the mill – They can be maintained constant during the grinding operation. 3. Group III - Circulating load, dwelling time of the material in the mill, feed rate – they affect capacity. 88
  • 83. TUMBLING MILL - GRINDING MEDIA Grinding occurs during tumbling action of the mill. The media used – Steel balls of varying sizes – 25 – 125 mm dia. They are made of Hadfield ( Mn ) steel, Cr steels, Cr-Mo steels. WC ( Tungsten Carbide ) / ceramic balls are also used – Costly but long life. Large pieces of ore are used as grinding medium in Autogenous grinding. Tumbling mills Ball mill, Pebble mill, Rod mill, Autogenous / Semi autogenous mills. Tube mill. 89
  • 84. 90
  • 85. 91 A BALL MILL
  • 86. 92 BALLS INSIDE A BALL MILL Balls range in size from 1-6 “. They are made of cast iron, forged steel or alloy steel. Larger balls help in coarse grinding .
  • 87. 93 BALL MILLS ( 1876) . They are rotating cylinders with liner and grinding balls inside. Grinding medium - steel / iron / WC balls. They are continuous machines ( 4-20 rpm) . Crushed ore is fed in one end through a feeder. The balls fall back onto the feed during rotation of the shell. Ground ore is discharged at the other end or through the periphery . They are used in closed system for maximum efficiency. Classified according to 1.shape (cylindrical, cylindroconical ), 2.method of discharging ground material (overflow discharge, grate / diaphragm discharge) or 3.method of grinding ( wet or dry). The mills have a shell and lining inside. They are fitted to a frame at the ends using trunnions.
  • 88. THEORY OF BALL MILLING ( ZONES IN A BALL MILL )- The path of the balls – circular section ( when balls are lifted ) + parabolic / near parabolic section ( when the balls are dropped ). Consider a ball at a distance ‘r’ from the center of the mill revolving at ‘n’ rpm . The ball leaves the circular path for a parabolic path when the centripetal component of gravity exceeds the centrifugal component of angular acceleration. i.e, when mv2 = mgcos α --------- 1 ---- r m - mass of the ball, v – its linear velocity, r – radius of the mill , g – acceleration due to gravity. 94 Path of a typical ball Forces acting on the ball at a distance ‘r’ from the center of the mill.
  • 89. THEORY OF BALL MILLING- But, v = 2 ∏ r n. Cos α = mv2 =4∏2n2r ------ -------- -------2 r mg g The locus of the points ‘E’ represents the beginning of the PARABOLIC PATH for different positions of the balls from the center to the periphery ( OBA ) . β = 3 α. From this, the locus of the points ‘F’ representing the end of the parabolic path is DCO. CO and BO correspond to ‘unstable equilibrium’. So, zone BM’C is a dead zone – no effective motion occurs here. 95 Path of a typical ball Forces acting on the ball at a distance ‘r’ from the center of the mill. Zones in a ball mill.
  • 90. 96 Zones in a ball mill. A photograph taken. There are 4 zones inside a ball mill. 1. an empty zone , 2. a dead zone, 3. a zone of circular path and 4. a zone of parabolic path. Crushing should take place along CD and nowhere else (Davis) It is NOT TRUE. 1.Crushing ALSO occurs when the balls roll on each other by slippage between ball layers. 2.Crushing ALSO occurs by attrition between the balls.
  • 91. 97 MOVEMENT INSIDE A BALL MILL Cascading- at low speed. Catarating – intermediate speed- grinding occurs. Centrifuging- at high speed.
  • 92. 98 CRUSHING ACTION IN A BALL MILL
  • 93. Nc =42.3(D-0.5) where Nc = critical speed (revolutions per minute) D = mill effective inside diameter (m) A mill is designed to achieve 75-80% of critical speed. Critical speed defines the velocity at which steel balls will centrifuge in the mill rather than cascade D Nc 2 30 3 24 4 21 8 15 12 12 99 The SPEED OF ROTATION (rpm) of the loaded ball mill is critical to success: too slow- the milling takes too long ; too fast - the contents (balls, ore and water) slam against the wall, inhibiting milling. Centrifuging grinding
  • 94. FACTORS AFFECTING SIZE OF PRODUCT FROM BALL MILL- It is important to fix the point where the charge, as it is carried upward, breaks away from the periphery of the Mill. This is called as “break point” or “angle of break” because it is measured in degrees. It is measured up the periphery of the Mill from the horizontal . There are four factors affecting the angle of break: 1. Speed of Mill 2. Amount of grinding media 3. Amount of material 4. Consistency or viscosity (for wet grinding) 100
  • 95. Feed end Discharge end Trunnion Lining Feed Inside the mill Ground slurry Trunnion 101 CYLINDRICAL MILL:- Not commonly used. The shell has cylindrical shape. The length of the cylinder = diameter of the mill. Three types depending on the discharge of the ground material. 1. Peripheral discharge ( central or peripheral ) mill - Discharge through screens in the cylindrical shell . Eg: Krupp mill. The screen wear is high and hence expensive. 2. Free Over flow mill- Simplest design. The ground material is discharged as overflow from the exit end of the mill, through Trunnion. Operates wet. 3.Grate ( diaphragm ) mill – The discharge end has grates. cylindrical protrusion used as a mounting and/or pivoting point.
  • 96. Overflow type ball mill also has a spiral inside the discharging end. The direction of the spiral is opposite to the running direction of cylinder body, which can prevent steel balls and ores from being discharged out of ball mill. CYLINDRICAL MILL 102
  • 97. OVERFLOW TYPE BALL MILLS 103
  • 98. 104 Grate or diaphragm discharge It is a low level forced discharge type grinding equipment. The discharge end of this mill has a slotted full diameter ( or partial diameter ) grate with discharge lifter assembly . This conveys the material to the discharge trunnion opening. The grate retains balls, tramp and coarse material. A gradient created between the feed opening and the discharge slots eases the flow of material. More balls can be used. So, more grinding area. Productivity of this mill is 10-15% more than that of an overflow type mill of same specifications.
  • 99. BALL MILL WITH – GRATE-DISCHARGE 105
  • 100. 106 BALL MILL WITH GRATE OR DIAPHRAGM DISCHARGE
  • 101. CYLINDROCONICAL MILL A cylindrical section connects an obtuse cone with a steep acute cone ( discharge end ) . Both ends have hollow trunnions. Feed is given through a feeder located before the trunnion at the charge end. A continuous stream of ground pulp pours out from the discharge lip ( located axially after the discharge end trunnion). Interior lining- cast iron or cast steel. The conical section at the charge end forces coarse particles and large dia. balls to the cylindrical portion. Maximum centrifugal effect is at the central cylindrical portion. Fine particles get ground in the conical sections by smaller dia. balls. This causes PREFERENTIAL GRINDING( larger particles by larger balls and smaller particles by smaller balls- result almost equal size particles). 107
  • 102. 108 CYLINDROCONICAL MILL Mill cross section classifier to remove fine powder Dry Hardinge mills are used in conjunction with pneumatic classifiers.
  • 103. HARDINGE MILL The peripheral speed of the cylindrical section and the conical sections are different – causes impact grinding in cylindrical section and abrasive grinding at conical section. 109 CYLINDROCONICAL MILL
  • 104. MULTIPLE COMPARTMENT CYLINDRICAL MILL:- It is also called a TUBE MILL . It has higher length to dia. ratio ( 5:3 ). It has TWO OR MORE COMPARTMENTS. Each successive compartment have sized balls. It is like different mills arranged in series. High reduction ratios are obtained. 110
  • 105. 111 SERIES OF BALL MILLS – MUFULIRA MINE GRINDING AISLE – ZAMBIA 1969
  • 106. 112
  • 107. ROD MILL It is a tumbling mill having a rotating cylindrical shell. 75-100 mm dia steel rods are the grinding media. They are laid parallel to one another. They cover the length of the mill. Size reduction of the ore is by line of contact between rods. The rods are kept apart by coarse particles. This causes preferential grinding of coarser particles. The mill is longer than its diameter – avoids jamming of the rods. 113
  • 108. Rod mills produce minimum fines ( Ball mills produce more fines). The ground product is concentrated by gravity or magnetic methods ( Finer ball bill products are concentrated by flotation method ) . Discharge can be- overflow discharge, end peripheral discharge or center peripheral discharge. The rods used wear fast and need replacement often. 114
  • 109. 115
  • 110. 116
  • 111. 117
  • 112. PEBBLE MILLS Cylindrical mills with porcelain / rubber linings. Grinding medium- porcelain pebbles. WHEN PEBBLES OF SAME ORE ARE USED TO GRIND- AUTOGENOUS GRINDING. Used for secondary grinding. It avoids contamination by iron linings. Liberation action is gentle. Less power consumption. There are two types 1. Tube mill and 2. Cascade and aerofall mill. 118
  • 113. 119 SEMI AUTOGENOUS GRINDING MILL - SAG SAG mills utilize grinding balls to aid in grinding like in a ball mill. It is generally used as a primary or first stage grinding solution. SAG mills use a ball charge of 8 to 21%. Attrition between grinding balls and ore particles causes grinding of finer particles. SAG mills are characterized by their large diameter and short length as compared to ball mills. The inside of the mill is lined with lifting plates to lift the material inside the mill, where it then falls off the plates onto the rest of the ore charge. SAG mills are primarily used at gold, copper and platinum mines with applications also in the lead, zinc, silver, alumina and nickel industries. The largest SAG mill is 42' in diameter, powered by a 28 MW (38,000 HP) motor AUTOGENOUS GRINDING MILL SAG . They are so-called due to the self-grinding of the ore. A rotating drum throws larger rocks of ore in a cascading motion . It causes impact breakage of larger rocks and compressive grinding of finer particles. Eg HADSEL MILL
  • 114. TUBE MILL ( Ball mill with different compartments)- They are long ( 5’ dia, 22 ‘ long) ball mills. Made narrow- to prevent breakage of the pebbles. Made longer- to compensate for the narrowness- to complete the grinding work over the length. Used in gold ore grinding. CASCADE AND AEROFALL MILL- Have larger dia. Dia/Length > 3. The lumps fall through a large distance. The feed and discharge are through trunnions. The mill rotates fast. High reduction ratio. Used for iron and gold ores. 120 Feed+ air to cyclone Drive
  • 115. 121 AEROFALL MILL-
  • 116. 122
  • 117. FACTORS AFFECTING OPERATION OF A GRINDING MILL- 1. SPEED OF THE MILL. Affects grinding efficiency and capacity. If the peripheral speed of the mill is very high i.e., > critical speed, it begins to act like a centrifuge and the balls do not fall back, but stay on the perimeter of the mill. NO WORK IS DONE BY THE MILL. If speed is very low- the grinding media will roll and slide over one another in a mass near the bottom of the mill- Cascading action. At intermediate speeds, grinding media is carried up to a certain extent. They fall on the material and grinding action takes place- Cataracting action. Ball mills operate at 55-85 % of critical speed. a) Cascading at low speed, b) Intermediate speed- grinding- cataracting action c) Centrifuging – high speed. 123 Nc =42.3(D-0.5)
  • 118. 2. LOAD OF GRINDING MEDIA AND MATERIAL. Ball mills should be slightly more than half full. The volume contributed by water and material is less compared to that contributed by the balls. 3. PULP LEVEL. Higher pulp level in a mill needs more power. Lower pulp level gives more degree of freedom of movement of the grinding media - more effective grinding. In an overflow discharge mill, the grinding media looses their kinetic energy when they fall back into the material to be ground. Grinding is less effective. 124
  • 119. 4. SIZE DESTRIBUTION OF GRINDING MEDIA. Should be near to close packing. The ratio between the balls of various sizes used should be proportional to the feed and discharge products. The proportion can be maintained with 1. By determining the wear rate of the grinding media and 2. By sizing analysis. The grinding media size is proportional to the feed size. If the size is small, they may get abraded and no grinding occurs. 125
  • 120. 5. SOLID LIQUID RATIO. Pulp dilution affects grinding efficiency and power input. Energy input is maximum for a pulp having 60-75 % solids. Fine powder in the mill is like a fluid. It is easily transportable in the mill. When 8-15% moisture is added, they form a stiff mud. Above 15% moisture content – it again becomes a fluid. 20-40% moisture- the material moves very efficiently. 40% moisture – pulp becomes watery and starts coating the grinding medium. 126
  • 121. SOLID LIQUID RATIO. For efficient use of energy in grinding – 1.ore particles should form a coating on the liners and the grinding media. 2.the pulp should be fluid enough for steady flow through the mill. Thin pulp - Solids settle, centrifuging occurs, coating on the grinding media is not possible- improper grinding . Thick pulp - Charge becomes pasty. KE is wasted to overcome the viscosity- ores particles get sandwiched between two grinding bodies – no proper grinding results. Optimum solid- liquid ratio:- Depends on coarseness of the ground ore, density of the ore. Helps to hold the crop load by friction, give desired lift during rotation of the mill and proper coating on the grinding media. For good , efficient , high capacity grinding: – Even distribution of the pulp, Clinging layer of particles on ALL METAL SURFACES within the mill. 127
  • 122. CHARGE VOLUME OF A GRINDING MILL- Volume of the charge – volume of material within the liners. It contains both the feed and the grinding media. It is calculated when the mill is at rest. Average volume is calculated. Weight of the charge is calculated with its bulk density. Ball Mills are normally run with charge volume < 45 %. This saves power consumption. Charge liner mill shell Mill dia D Side view of mill at rest H 128
  • 123. MOTOR SIZE- ( for a particular grinding operation or a particular plant ). The power needed to grind a given FEED SIZE to a given PRODUCT SIZE- W = 10 * Wi * [ ( 1/√P ) – ( 1/ √F ) ] – power calculated to drive the mill to grind the feed from one size distribution to a finer size distribution. W = Power consumption- KWH / metric tonne for wet grinding, closed circuit , P > 70 μ. P = screen opening size in microns, through which 80% product passes. F = size of the screen opening ( microns ) through which 80% of the feed passes. Wi = The Bond Ball Mill Work index ( 1952 )- is a measure of the resistance of the material to crushing and grinding . A Bond Ball Mill Work Index test may be required for the design of a new mineral processing plant . It is determined by Bond Ball Mill Grindability test in a lab using a Bond Mill. Grindability is the number of net grams of ( screen ) undersized product per revolution of the mill . Corrections are done if P is less than 80% passing 70μ size particles. 129
  • 124. CALCULATION OF MILL SIZE MATCHING TO REQUIRED POWER. What should be the mill size to draw power W. Power input to maintain the condition in the figure is KW = [ M * C * Sine α ( 2 πN ) ] / 12.400 M - wt of charge ( metric tonne ), C - distance of center of gravity of charge from center of mill ( meters ), α - dynamic angle of repose, N - mill speed ( r.p.m ). α N rpm Consider a mass m resting on an inclined plane. If the angle of inclination is slowly increased, a stage will come when the block of mass m will tend to slide down. This angle of the plane with horizontal plane is known as angle of repose α α 130
  • 125. OPTIMUM SIZE OF GRINDING MEDIA. Major opposing factors- 1. As the size of the grinding media increases, lesser surface area, the pressure between the surfaces decreases – causes breaking of larger particles. 2. As the size of the grinding media decreases, more surface area , the pressure between the surfaces increases- causes grinding of smaller particles . Other factors – 1. Harder ore lumps - larger grinding media 2. Larger dia mill - needs smaller grinding media to break particles of a given size. 3. Faster mill speeds - needs smaller grinding media to break particles of a given size. For most efficient grinding – the initial charge of the grinding media should have the size distribution in a complete range from maximum to minimum size. 131
  • 126. CONSUMPTION OF GRINDING MEDIA. Due to 1. abrasion of the media surface by contact with the material , 2. corrosion of the freshly exposed media surface. Major grinding media consumption is by Corrosion, especially in wet grinding. Average Consumption of media kg/tonne of feed. 132 Mill Dry Wet Rod mill 0.15 0.45 Primary ball mill 0.15 0.45 Secondary ball mill 0.07 0.40
  • 127. RECENT TRENDS IN CRUSHING AND GRINDING To improve the art of crushing and grinding for 1. better grain liberation, 2. minimum overgrinding, 3. reducing power consumption, 4. reducing capital and operation costs. Improvements- Development of 1. Autogenous and semiautogenous grinding machines, 2. Reduced cone oscillations in cone crusher – Rapid rotation of unbalanced load ( and not the motion given by the eccentric) gives improved efficiency. 3. Centrifugal crusher ( very fine size possible) 4. High pressure grinding rolls. 133
  • 128. RECENT TRENDS IN CRUSHING AND GRINDING. HIGH PRESSURE GRINDING ROLLS (HPGR) - Utilizes compression breakage of a particle bed. Energy efficient inter-particle breakage occurs . Reduction ratio obtained in a single pass is higher than that obtained in conventional rolls crusher. ADVANTAGES:- Reduces grinding strength , Improves leachability due to micro-cracking , Reduces energy requirements ( 20-50% more efficient than conventional mills). INCREASES LIBERATION Via RECOVERY. APPLICATIONS:-Tertiary or quaternary crushing stages, Pebble crushing, Open or closed circuit operations. 134
  • 129. The feed material is exposed to very high pressure for a short time. The high pressure – 1.forms micro-cracks in the feed particles , 2.generates a substantial amount of fine material, 3.promotes micro fracturing of particles in product ( better leaching ) HPGR has two counter-rotating rolls – one fixed and one floating . The feed is introduced to the crushing zone. Hydraulic cylinders apply very high pressure to the system in a controlled way. It causes inter-particle comminution as the feed travels between the two rolls. The basic operating principle behind HPGRs makes them very energy efficient: HIGH PRESSURE GRINDING ROLLS (HPGR) 135
  • 130. Hydraulic pressurising cylinders Nitrogen-filled bladder accumulators Studded Roll Feed Chute Two variable speed drives HIGH PRESSURE GRINDING ROLLS (HPGR) Studded Roll 136
  • 131. Every single particle of the input material undergoes extreme acceleration in the twin-chamber rotor . Each particle is subsequently thrown against the fixed impact wall . The impact wall can consist of an anvil ring or a rock shelf. The large size of the rotor and housing eliminates the risk of clogging. The crushing is influenced and optimized by rotation speed. ROTOR CENTRIFUGAL CRUSHER 137
  • 132. ROTOR CENTRIFUGAL CRUSHER 138
  • 133. ATTRIBUTES OF COMMINUTION. 1. Shape of the comminuted particles, 2. Effect of cleavage , 3. Sizes present in the comminuted product, 4. Relative abundance of particles of different sizes. THERMODYNAMIC DEFINITION OF CRUSHING / GRINDING EFFICIENCY. Energy is spend - as Kinetic energy. Energy recovered as - Potential energy ( surface energy ) + heat + sound. Only the potential energy is useful. Efficiency of communition process- ratio of the surface energy produced to the kinetic energy expended. 139
  • 134. 140
  • 135. 141 The screen is a surface with many apertures or holes, usually with uniform dimensions. Particles of different sizes are presented to that surface . The undersize material will pass through and oversize materials will be retained. Thus the incoming material is sorted out according to aperture size of the screen. Further processing depends on the size of ore. SIZING.
  • 136. 142 WHAT IS THE NEED FOR SCREENING? (a) SIZING OR CLASSIFYING - to separate particles by size, usually to provide a downstream unit process with the particle size range suited to that unit operation; (b) SCALPING - to remove the coarsest size fractions in the feed material, usually so that they can be crushed or removed from the process; (c) GRADING - to prepare a number of products within specified size ranges. This is important in quarrying and iron ore, where the final product size is an important part of the specification; (d) MEDIA RECOVERY - for washing magnetic media from ore in dense medium circuits; (e) DEWATERING - to drain free moisture from a wet sand slurry; (f) DE-SLIMING OR DE-DUSTING - to remove fine material, generally below 0.5 mm from a wet or dry feed; and (g) TRASH REMOVAL - usually to remove wood fibres from a fine slurry stream.
  • 137. SIZING. Separation of particles according to their size. Methods – Screening ( Sieving ) , Classification ( Elutriation - process for separating lighter particles from heavier ones using a vertically directed stream of gas / liquid ) and microscopic methods. Screening – The crushed ore is spread on a mesh of particular aperture size and shape. The material gets separated into oversize and undersize. A size distribution is obtained with different meshes with decreasing aperture sizes. 143
  • 138. A FACTORS AFFECTING THE SCREENING EFFICIENCY. SCREENING EFFICIENCY- Weight of grains of given size in the screened product / their weight in the supplied feed. 1. PARTICLE SHAPE. Particles are irregular in shape They are assumed to be spherical / cubic. Consider - 1. A square meshed sieve. 2.It has a linear distance ‘A’ between successive parallel wires. 3. Surface ‘S’ in the square = K / ( A.ρ ) K= constant 60 for cubic, 50 for oblong, 240 for laminar etc. ρ = Specific gravity of the mineral. 144
  • 139. 145 PARTICLE SHAPES
  • 140. 146 1. PARTICLE SHAPE. Most granular materials processed on screens are non- spherical. Spherical particles pass with equal probability in any orientation. Irregular-shaped near-mesh particles must orient themselves in a direction that permits them to pass. Elongated and slab like particles will present a small cross- section for passage in some orientations and a large cross- section in others. The extreme particle shapes therefore have a low screening efficiency.
  • 141. 147 2. SIZE OF SCREEN OPENING. The capacity of a screen is proportional to the screen aperture. The chance of a particle passing through the aperture is proportional to the percentage of open area in the screen material ( i.e., the ratio of the net area of the apertures to the whole area of the screening surface ). Open area decreases with the fineness of the screen aperture. Use of very thin and fragile wires for deck construction increases the open area of a fine screen. Near mesh size particles tend to peg or plug the apertures and reduces the screen efficiency.
  • 142. 3. RELATIVE SIZES OF PARTICLE AND OPENING. It controls the passage / non passage of the particles. A small particle can fall easily through the sieve opening. As its size approaches that of the opening, it becomes increasingly difficult to fall through the opening. “a” screen aperture, “d” particle diameter. If the particle strikes the screen NORMALLY, it will not go undeflected through the opening. To go undeflected, the particle should be at the center of the opening. For wires of small cross section, probability of passage ‘p’ = [ ( a-d ) / a ]2. 148 Passage of particles through a wire screen- normal incidence- Case1. Wires have small cross section compared to the openings.
  • 143. RELATIVE SIZES OF PARTICLE AND OPENING. For wires of large cross section, probability of passage “p” = [ ( a-d ) / ( a+b ) ]2. 149 Passage of particles through a wire screen- normal incidence- Case2. Wires have larger cross section .
  • 144. 150 4. ANGLE OF INCIDENCE OF PARTICLES ON SCREENS. It is an important parameter. The particle should fall with its minimum cross section , normal to the aperture . The fall should be slow. As the deviation from normal increases, the chance of passage of the particle decreases. In practice, particles fall in all angles at very high speed. The screen angle also affects the speed at which particles are conveyed along the screen, dwell time on the screen and number of opportunities particles have to pass through the screen.
  • 145. 7. MOISTURE CONTENT. Very dry particles / wet pulp can be easily screened. Difficult to screen dry particles with small amount of water . Dampness causes particles sticking together or clinging to screen surface. 151 5. PERCENTAGE OF SCREEN AREA OPENING. More the area of opening, faster will be the rate of screening 6. FRICTION OF THE SIEVE. Friction of the screen makes bunches of fine, dried particles. It is due to static electricity. Several undersized particles make a bunch and find difficult to pass through the screen. They may be wrongly reported as oversize particles. Bunching increases with temperature- screening is to be done in cool rooms.
  • 146. 8. SOLUBLE SALTS. Soluble salts adhering to particles increase effective cross section of the particles- wrong report of sieve analysis. 9. SPREADING OF FEED OVER SCREEN AREA. If the feed is spread wider on the screen, it is easier for the particles to pass through the openings. 10.CORROSION OF SCREEN MATERIAL. This produces roughness on the sieve and changes the opening size. Stainless steel sieves may be used. 152
  • 147. 153
  • 148. WHEN AND WHERE? Material for size analysis is collected from the crushing section, the grinding section or closed circuit return / discharge . Material from the crushing section - to assess the work done by the crusher and the work to be done by the subsequent grinder. Material from the mill discharge of grinding section – to assess the size distribution of discharged particles. Material from closed circuit return- to asses sizes of materials returned for further grinding. Material from closed circuit discharge- to assess whether the valuables are properly liberated for further concentration. 154
  • 149. NEED. Lab sizing tests are done to check 1. Progress of the material at various stages of treatment, 2. The efficiency of power and 3. The effect of grinding on the recovery of values from the ore. Lab. screening is normally done upto 40 microns ( making sieves with accuracy below this is difficult). The apertures ( meshes ) of sieve- they are reference points telling whether particles will pass through them or not. This depends on the cross sectional area of the particles. A screen- an assembly of various apertures to test many particles at the same time. 155
  • 150. THE SCREEN. Screens are circular shells of brass – 8” dia and 2” ht. Screen cloth is at the bottom of the shell. Screens nest in each other in order of decreasing size. A complete set has a cover and bottom pan. 156
  • 151. THE TECHNIQUES. Dry or wet screening is done in a laboratory. Wet screening is done to remove the unwanted particles, remove the undersized particles, dissolve the dried-on salts and disperse adhering clay. Wet screening- Dry the sample and weigh it. Disperse it in water having sodium silicate ( water glass Na2SiO3 – binds colloidal material). Wash gently on the finest screen of the series. Collect the undersize in a bowl. The oversize is dried and screened using desired set of sieves. 157
  • 152. 1. HAND SCREENING. Three screens are assembled ( NEST of SIEVES )- the coarsest at the top and the finest at the bottom. The sieves should not have any defect and they are brushed to clear the apertures before assembly. The final undersize product is collected in a pan, tightly fitting the third screen. The sieves should closely fit to prevent loss of fines during sieving. A sample of 200 gms is loaded in the upper sieve. For good screening – ideal load is one particle deep. normally upto four particles deep are loaded with good accuracy. 158
  • 153. 159 The assembly is held in the right hand and struck against the palm of the left hand. The sieve assembly is rotated by 60O and the direction of shaking is changed every 30 seconds. The operation is repeated for a given length of time. The time of sieving depends on the nature of the material. ( Eg:-friable materials should not be screened with shock for longer period- they disintegrate). The undersize is removed and sieve analysis is carried out.
  • 154. 2. AUTOMATIC SCREENING. Normally used for lab. work. Upto 6 standard screens ( sometimes 12 ) with the bottom pan are assembled on a framework of a mechanical shaker. Normal operating time – 5-20 minutes. In the Tyler machine, the material on the screen is given a continuous circular motion. A blow is given to the nest of sieves once for each revolution. MOST SATISFACTORY SCREENING RESULT – EACH SCREEN SHOULD CONTAIN MATERIAL REDUIRED TO FORM ONE LAYER OF ONE PARTICLE DEEP. Limitation- Fine particles may adhere to coarse particles or to each other through electrostatic action. It can be prevented by wet-cum-dry screening. 160
  • 155. 161
  • 156. 162 TYPES OF SCREENS. Screens are made of 1)sheet metal with round or square holes. The apertures will be flaring out and down ward for easy fall of particles. 2)woven or welded rods/wires ( coarse sieve- steel wire. Fine sieve- brass (95% Cu, 5% Zn ) / bronze (88% Cu,12%Sn ) wire. The mesh – defined as the number of apertures ( holes ) per unit length ( inch or cm). Different countries use wires of different thicknesses to make the sieve. This causes difference in the aperture size of the screens used in different countries. So standards are employed. U.S.A - TYLOR, American Standard Testing of Materials (ASTM E 11-70). UK - British standard ( B.S 410-62, 410-69), Institution of Mining and Metallurgy (IMM- London). FRANCE - AFNOR X 11-501 (Association Française de Normalisation). GERMANY- DIN 4188-1957 (Deutsches Institut für Normung - The German Institute for Standardization) . INDIA - IS 460-1962 .
  • 157. 163 LOGOS OF VARIOUS INTERNATIONAL STANDARDS.
  • 158. TYPES OF SIEVES 164 Typical openings in laboratory sieve series Sieve size (mm) BSS [6] Tyler (approx) US (approx) 4.75 - 4 4 3.35 5 6 6 2.81 6 7 7 2.38 7 8 8 2.00 8 9 10 1.68 10 10 12 1.40 12 12 14 1.20 14 14 16 1.00 16 16 18 0.853 18 20 20 0.710 22 24 25 0.599 25 28 30 0.500 30 32 35 0.422 36 35 40 0.354 44 42 45 0.297 52 48 50 0.251 60 60 60 0.211 72 65 70 0.178 85 80 80 0.152 100 100 100 0.125 120 115 120 0.104 150 150 140 0.089 170 170 170 0.075 200 200 200 0.066 240 250 230 0.053 300 270 270 0.044 350 325 325 0.037 440 400 400 The aperture of a standard mesh varies from one another by 1.414 [ i.e (2)1/2 ]. Eg :- in Tyler series- Mesh no 200 – aperture width 74 microns 150- 74 * 1.414 = 104 μs. ( next larger size) 250- 74 / 1.414 = 53 μs.
  • 159. 165 SIEVE ANALYSIS
  • 160. TYPES OF SCREENS. IMM- - wire dia = width of aperture- gives 25% screening area. ASTM - wires are thinner – gives 30% screening area. BS - still thinner wires- gives 40% screening area. GREATER THE SCREENING AREA – HIGHER THE SCREEING RATE ( screens will be of less strength). Number of apertures in a screen is INVERSELY PROPORTIONAL to ( Aperture ) 2. 166
  • 161. PRESENTATION OF SEIVE ANALYSIS. 167 Size m mm Actual % Cummulative * % +13.33 -13.33 + 9.423 -9.423 + 6.680 -6.680 + 4.699 -4.699 + 3.327 -3.327 + 2.362 -2.362 + 1.651 -1.651 + 1.168 -1.168 + 0.833 -0.833 + 0.589 -0.589 + 0.417 -0.417 + 0.295 -0.295 + 0.208 -0.208 + 0.147 -0.147 + 0.104 -0.104 + 0.074 -0.074 0.74 2.60 11.41 32.12 18.34 11.05 7.13 4.83 3.33 2.30 1.71 1.33 0.94 0.70 0.52 0.33 0.62 100.00 0.74 3.34 14.75 46.87 65.21 76.26 83.39 88.22 91.55 93.85 95.96 96.89 97.83 98.53 99.05 99.38 100.0 *increasing by successive addition.
  • 162. PRESENTATION OF SEIVE ANALYSIS. 168 % retained Size, mm % retained By Individual size-grade Size, mm Cumulative weight charting . Direct charting ( weight –frequency plot).
  • 163. SUB SIEVE SIZING. Normally sieving is done in labs upto 70 microns size. There may be a lot of material unsized. Sub sieving is done to size particles less than 70 microns in size. Methods used are SEDIMENTATION, ELUTRIATION, INFRA SIZING, PERMEABILITY METHOD AND MICROSCOPIC MEHTODS. 169
  • 164. SUB SIEVE SIZING. 1. SEDIMENTATION. It is based on the FALLING RATE of SMALL PARTCLES through STATIC LIQUIDS. The sample is suspended in a fluid medium. Suitable agitation is given. Now it is allowed to settle for a fixed time. The settling action is stopped by withdrawing ( siphoning out ) the fluid medium after the fixed time. The sediment is again subjected to the above operations. Nearly 20 or more such cycles are carried out. In each cycle weight of the sediment is reported. Sedimentation method is used to size down to 5 -2 microns. 170
  • 165. 171 SUB SIEVE SIZING. 2. ELUTRIATION. The fall of a particle due to its mass is retarded by the friction between the particles and the rising column of fluid. This depends on the total surface of the particle. When a large number of spheres having the same specific gravity move in a rising column of water – the smaller ones rise, the medium ones dance in a diluted layer, the larger ones fall through the water. This behaviour of the spheres depends on the chances of collision and Kinetic energy gained during rising or falling. This principle is used to separate out particles.
  • 166. 172 Re = ( 2 r Vm Δ’ ) /μ, = Reynold’s number - a dimensionless number that gives a measure of the ratio of inertial forces to viscous force Vm= ( 2/9 ) * [( Δ - Δ’ ) r2 g ] / μ. Δ = Sp gr of particle, Δ’ = Sp gr of fluid medium, r = radius of the particle, μ = viscosity of the medium, Vm = terminal velocity of the particle. g = gravitational acceleration IN THE PROCESS OF ELUTRIATION- Particles falling in a rising fluid can be classified into two sizes. HOW? The fluid is rising with a certain velocity. The particles having terminal velocities > this velocity will settle at the bottom of the sorting column. The particles having terminal velocities < this velocity will be lifted to the top of the sorting column. This will be carried away to the next tube containing sorting liquid. The process repeats. Terminal velocity of a fluid is calculated from Stoke’s law. STOKE’S LAW Theforce that retards a sphere moving through a viscous fluid is directly proportional to the velocity of the sphere, the radius of the sphere and the viscosity of the fluid.
  • 167. 173 Keep the volumetric flow rate of the rising fluid constant. Now the velocity of the rising fluid in the columns depends on the column diameters. Narrow dia column gives high velocity - This allows coarser particles to settle. Larger dia columns gives lower velocity - This allows finer particles to settle. Columns of decreasing dias. are assembled in a series. They separate particles of different sizes.
  • 168. ELUTRIATION. The laboratory BLITHE ELUTRIATOR consists of i) Water supply at constant head, ii) Flow rate controller for the water rising through the elutriator, iii) A sorting tube with smooth and parallel sides and iv) Separate vessels ( receptacles ) to catch the overflowing and under flowing particles. 174
  • 169. 3. INFRASIZING . This can size materials that are finer than those handled by sedimentation or elutriation. The process- A current of air is sent with miniumum turbulence through a series of tubes. The tubes have increasing diameters. A 50 gm sample ore is introduced into the air stream. The infra-sizer and the sample should be dry. Electrostatic effects on the particles is reduced / removed by using conducting rubber in the flexible connections. 175
  • 170. 4. MICROSCOPIC MEASUREMENTS. A metallurgical microscope is used for this purpose. The microscope has calibrated stage. The ore particles are sprinkled or dispersed on the stage. Linear measurement of each particle is done separately. Average size of particle is calibrated in the lot of material sprinkled. Can make actual measurements upto 0.5 micron level. Even finer particles can be measured with microscopes having better resolution. This method is unsuitable if sizes of particles differ very much since - 1. larger particles may cover finer particles and may not be visible 2. all particles may not be in focus, 3. fine particles may stick to larger particles. 176
  • 171. 177 MICROSCOPIC MEASUREMENTS. Place an engraved linear scale ( Ocular - glass ) in the eyepiece.Keep a micrometer on the stage. Both the scales are correlated for a particular magnification. Remove the stage micrometer. Keep the specimen to be measured. Measure with the ocular.
  • 172. 178 .
  • 173. COMMINUTION OF ORES INDUSTRIAL SCREENING. INTRODUCTION. Two methods- Screening and Classification. Screening - used to screen particles of several inches in size to particles as fine as 0.1 mm. Normally used to size coarser than 20 mesh screens. Classification - used to classify particles of 2-3 mm sizes to 0.02-0.03 mm. Normally used to size finer than 35 mesh screens. SCREENING:- Done to 1. prevent oversized particles from entering a machine, not suitable for them. 2. to separate undersized particles from the feed to a crushing machine which is meant to crush larger pieces, 3. to separate minerals/ ores into specified sizes and 4. to prepare a correctly sized feed for a given, subsequent concentration process. 179
  • 174. SCREENING. Screening surfaces are of three types- 1.Parallel rods- Made of steel bars, rails, cast iron or wood. They have tapered section. 2.Punched plates- Made of sheet steel. Circular/square/slot like holes are punched. Square apertures give more screening area. Circular holes are better for coarse particles. 3.Woven wire screening surface- Carefully gauged wires ( Steel, copper, bronze ) are woven or placed in two perpendicular directions equally and welded. PICS 180
  • 175. TYPES OF INDUSTRIAL SCREENS INDUSTRIAL SCREENS WET & DRY. FIXED SCREENS MOVING SCREENS GRIZZLIES ROLL SCREENS FLAT SCREENS PUNCHED SCREENS SHAKING CONVEYING SCREENS SCREENS REVOLVING SCREENS VIBRATING SCREENS TROMMELS ( REELS ) MECHANICAL ELECTRICAL 181
  • 176. 182 SCREENING - FIXED SCREENS. 1.GRIZZLY. It is used to protect primary crushers against overload. They are made of steel or wooden bars having a rectangular cross section. The bars have taper from feed end to discharge end. The bars are held together by cross rods or by bolts. The stationary grizzly is set at a slope of 25-50o – helps in the free slide of the ore. VARIANTS- 1. Moving bar Grizzly- Alternate bars rise and subside . Helps in the forward movement of the material with TURNING OVER. 2. Roll grizzly- A series of grooved rollers are driven in the same direction. The rollers remove oversized lumps. 3. Grizzlies can be vibrated mechanically or electrically or by impact of falling ore lumps.
  • 177. 183 VIBRATING GRIZZLY
  • 178. PICS 184 2. ROLL SCREENS. A bank of rolls are arranged in the form of a cascade. They provide square apertures between adjacent rolls. Used to screen finer sizes ( 10-100 mm) of HARDER ORES. Costly, Less efficient. 3. FLAT SCREENS. They are inclined- helps in the flow of material. They are given periodic motion – helps in rolling /bouncing of the ore lumps . Lumps / particles screened depend upon 1. length of the screen ( increased length, better performance ), 2. slope of the screen ( increased slope, lesser particles are screened ), 3. amplitude of vibration ( should be just sufficient to clear the screen ), 4. frequency of vibration , 5. depth of particle bed on the deck ( 5-6 particles deep for optimum screening efficiency).
  • 179. Shells shaft 185 SCREENING – MOVING SCREENS. 1.TROMMELS ( REVOLVING SCREENS-REELS ) . They are rotating cylindrical shells of screen material ( punched sheet ). They are 3-4 ft in dia and 5-10 ft long. The material to be screened flows into the shell. The shell rotates. The shell is kept in an inclined position. The particles move towards the discharge ( lower ) end. The undersize particles are collected at the bottom. Two types of operation. 1. Cylinders are in one line-finest screen first, coarsest last. The first cylinder is fed with the crushed ore. Very fine particles are separated. The coarse oversize is fed to the next screen and the process continues. 2. Compound ( concentric ) trommel. Screens are made to rotate on a single shaft. The coarsest screen is the innermost and the finest screen is the outermost.
  • 180. TROMMELS – STATIONARY AND MOVING
  • 181. 187 COMPOUND TROMMEL TROMMEL IN SERIES
  • 182. 188 2. FLAT SCREENS. Most used screens in mineral dressing – can screen medium coarse and fine particles ( 250 mm and 0.2 mm). They have one or more decks. They work at low slopes. The screens are given reciprocating motion – straight line, perpendicular, circular or oblique movements. Rapid , low amplitude vibrations give higher capacity and efficiency. Problems to be taken care of in designing the vibratory screen. 1.Production of suitable reciprocating motion, 2.Even transmission of this motion to the whole screen, 3.Nontransmission of this motion to screen supporting structure, 4.Dust proofing vital parts, 5.Ease of replacement of worn screens.
  • 183. 189 FLAT SCREEN VIBRATION ISOLATORS
  • 184. 190 3. VIBRATING SCREENS Have one or more decks working at low slopes. Used for medium coarse and fine screening ( 250 mm and 0.2 mm) Screens are given a vibratory motion. Rapid low amplitude vibration – higher capacity and efficiency. a) ELECTRICALLY VIBRATED SCREENS. They use electromagnetic devices. A stationary electromagnet sets up a reciprocating motion. Amplitude is controlled by regulating the voltage given to the magnetic coils. The screening units may have one or two screening surfaces. Eg:- Hum-mer screen. Jeffry-Traylor screen.
  • 185. SCREENING - VIBRATING SCREENS. b) MECHANICALLY VIBRATED SCREENS. They impart straight line motion to circular orbit spinning. PLAT – O SCREEN- unbalanced flywheel induces mechanical vibration. TYLER-NIAGARA SCREEN – The mechanical vibrations are provided by eccentric motion. SYMONS SCREEN- It provides a gyratory motion generated by an unbalanced weight. The screen is operated in horizontal position. 191
  • 186. The material gets lifted and thrown forward when the screen moves to the front. When it moves back, the material is tossed in air. This causes largest particles on top and allows smaller ones to pass through the mesh. 192 SCREENING - SHAKING SCREENS. They have plane surface. They are used to sort dry and soft materials –normally for coal preparation. The screen is hung by chains. Shaking is done sidewise or endwise.
  • 187. 193 BLINDING of SCREENS by 1) hard,near size particles wedging in apertures, 2) soft , sticky material filling up the apertures. Cleaning at regular intervals is recommended.
  • 188. OPERATING CHARACTERISTICS OF SCREENS SCREENING CAPACITY ( tonnes / sq meter/ mm screen aperture / day) Depends on 1)area of the screening surface, 2)size of the opening, 3)specific gravity, moisture content, temperature, proportion of fines etc of the ore. 4)type of screening mechanism used. . Capacity decreases with increasing oversize fraction. Capacity is more for oblong apertures than square apertures. 194 Type of screen capacity range (ore) (tonnes/sq m/mm aperture/day) Grizzlies 10-50 Trommels 3-20 Shaking screens 20-80 Vibrating screens 50-200
  • 189. SCREENING EFFICIENCY. Is a measure of the effectiveness of the actual screening operation, as compared to the perfect screening operation. Efficiency E = % recovery of the true undersize material in actual undersize product = ( 10,000) U / uF. U= tonnage passing through the screen for each F tonnes of feed. u= % undersize in the feed ( from Lab screen result ). Limitations of the formula- It does not take into account- 1.particles that are JUST FINER than a GIVEN SCREEN OPENING are difficult to screen out. 2.particles much finer than the screen opening are easily screened out. So, same screen, on different feeds may show different efficiencies. Screening efficiency is related to dwelling time of the passing material and the opening available during its passage. 195
  • 190. Increased by 1. Increased % of the screen openings to passage of the undersized particles, 2. Smoothness of screen wires, 3. Suitability of aperture shape to the average particle shape and 4. Increased transit time. Adversely affected by 1. Increased rate of feed, 2. Increased % of near mesh grains, 3. Increased thickness of bed, 4. Increased moisture content and 5. Lack of response of screening surface to vibratory motion provided. 196
  • 191. OPERATING COST. Very low compared to other operations of mineral dressing- No operating cost for stationary screens, Power cost for moving screens, Low labour charges, Cost of replacing screens. For good screening operation- 1. There should be good tension on the screen in the frame, 2. The combined effect of vibration, speed and amplitude must be optimised, 3. There should be low moisture content in the feed, 4. The vibrating strokes should be equally distributed in all parts of the screen, 5. Feed should be spread entire width of the screen. 197
  • 192. 198